1. Field of the Invention
The present invention relates to a variable valve system for changing valve timing and lift amount continuously or stepwise in a range from lower revolutions to higher revolutions of an internal combustion engine.
2. Description of the Related Art
Conventionally, a variety of valve systems have been disclosed for changing the valve timing (including valve opening angle and phase) and lift amount in two steps between lower revolutions and higher revolutions of the internal combustion engine.
For example, one of such variable valve systems is provided with a swing arm that is caused to swing selectively by either a cam for lower revolutions or a cam for higher revolutions. The cam for lower revolutions opens and closes an intake or exhaust valve at smaller valve opening angles with smaller lift amounts, whereas the cam for higher revolutions opens and closes the valve at larger valve opening angles with larger lift amounts. FIG. 18 shows a relationship between valve timing and lift amount with regard to the intake (right) and exhaust (left) valves of this system. For lower revolutions of the internal combustion engine, as indicated by a solid line of FIG. 18, the valve opening angles and lift amounts of the exhaust and intake valves are set to smaller values respectively in order to generate swirl in intake air. This results in an increase in torque for lower vehicle speeds as well as a substantial reduction in fuel consumption. For higher revolutions of the internal combustion engine, as indicated by a broken line of FIG. 18, the valve opening angles and lift amounts of the exhaust and intake valves are set to larger values respectively, which results in a substantial increase in intake air volume and high speed outputs.
Another variable valve system includes a displacement device with helical splines, into which a cam shaft for intake valves is spline-fitted. The cam shaft has cams for pressing valve lifters of a directly-hitting type. The displacement device rotates the cam shaft by a predetermined angle, thereby making a distinction between lower revolutions and higher revolutions. This system changes valve timing phase and leaves valve opening angle and lift amount unchanged. FIG. 19 shows a relationship between valve timing and lift amount with regard to the intake (right) and exhaust (left) valves of this system. For lower revolutions, as indicated by a solid line of FIG. 19, the valve timing phase of the intake valves is shifted towards that of the exhaust valves, increasing the amount of overlap between the intake and exhaust valves. Thereby the intake valves are closed at an earlier stage, which results in an increase in torque for lower vehicle speeds as well as a substantial reduction in fuel consumption. For higher revolutions, as indicated by a broken line of FIG. 19, the valve timing phase of the intake valves is shifted away from that of the exhaust valves, reducing the amount of overlap between the intake and exhaust valves. Thereby the time period during which intake air flows into the internal combustion engine is prolonged, which results in an increase in high speed outputs.
In comparison with a generally employed non-variable valve system, the aforementioned conventional variable valve systems significantly improve various characteristics including torque, outputs, fuel consumption and cleanliness of exhaust gas. A dashed line of FIG. 7 indicates the torque characteristics of internal combustion engines equipped with the conventional variable valve systems, whereas a broken line of FIG. 7 indicates the torque characteristics of an internal combustion engine equipped with the generally employed non-variable valve system. It is obvious that the torque level achieved by the former is well above that achieved by the latter over the entire revolution range. Although there may be a slight difference depending on respective setting conditions, it is considered that the conventional variable valve systems can reduce fuel consumption by a maximum of 8 to 10%.
However, the conventional variable valve systems have the following drawbacks.
(1) The conventional variable valve systems both change valve timing or lift amount only in two steps between lower revolutions and higher revolutions. It is therefore difficult to execute control precisely depending on various operating conditions of the internal combustion engine. As indicated by the dashed line of FIG. 7, the torque characteristic curve may have a trough point between lower revolutions and higher revolutions. Such a trough point is likely to appear especially in those setting conditions which aim at improving the torque characteristics for higher revolutions, e.g., 8000 rpm. PA0 (2) There may be a contradictory problem raised by changing only the valve timing phase with the valve opening angle and lift amount remaining unchanged. If the valve opening angle is set to a large value to improve outputs for higher revolutions, the torque for lower revolutions deteriorates, causing an unstable idling state. To the contrary, if the valve opening angle is set to a small value to improve the torque for lower revolutions, outputs for higher revolutions deteriorates. PA0 (3) The contradictory problem as described above can be solved by selectively operating a cam for lower revolutions, which is used for smaller valve opening angles and lift amounts, and a cam for higher revolutions, which is used for larger valve opening angles and lift amounts. However, since it is necessary to provide one valve with two cams and two or three arms, the overall construction becomes complicated and less compact. In order to selectively operate the cams, there is generally employed a device for displacing a pin by applying a high hydraulic pressure thereto. However, such a device lacks precision and reliability, i.e., it is unable to witch the cams smoothly in a single operation cycle, causes noise when switching the cams, or partially wears out. In addition, a high hydraulic pressure source is required to quicken operational response in switching the cams. PA0 1) A swing arm having one end which is pivotally supported by a rocker shaft, the other end which is provided with a valve engaging portion, and an intermediate portion which is provided with a roller mechanism equipped with the cam contacting element. PA0 2) A swing arm having one end which is pivotally supported by a pivot, the other end which is provided with a valve engaging portion, and an intermediate portion which is provided with a roller mechanism equipped with the cam contacting contacting element. PA0 3) A rocker arm having one end which is provided with a roller mechanism equipped with the cam contacting element, the other end which is provided with a valve engaging portion, and an intermediate portion which is pivotally supported by a rocker shaft. PA0 a) In the case where the cam contacting element is a semi-cylindrical body comprising a semi-cylindrical surface that slides on the seat and a substantially flat contacting surface that contacts the solid cam, semi-cylindrical surfaces of the plurality of cam contacting element may be gradually varied in radius one another so as the plurality of cam contacting element to be different in thickness. PA0 b) In the case where the cam contacting element is a semi-cylindrical body comprising a semi-cylindrical surface that slides on the seat and a substantially flat contacting surface that contacts the solid cam, contacting surfaces of the plurality of cam contacting element may be gradually varied in height one another with the radius of semi-cylindrical surfaces thereof being constant so as the plurality of cam contacting element to be different in thickness. PA0 i) Both ends of the seat are closed by a closing wall on which both end surfaces of the cam contacting element abut. PA0 ii) An engaging recess formed in a longitudinally substantially central portion of the seat swingingly engages an engaging protrusion formed on a longitudinally substantially central portion of the cam contacting element.